In the oil and gas industry, and especially in subsea or other underwater well drilling procedures, it is well established practice to employe an annular seal assembly, referred to as a packoff, between adjacent concentric wellhead elements, such as the wellhead housing and the casing hangers that support the casing strings in the well, to pressure seal the annuli between these elements. For many years these packoffs have included elastomeric or other non-metallic annular seal elements that, when energized into tight contact with the opposed wellhead and hanger surfaces, provided the requisite pressure barrier. However, the growing trend towards drilling deep wells into relatively high pressure strata, and the frequency encountering in these wells of hydrogen sulfide or other corrosive gases, has led to the development of packoffs with all metal seal elements to establish a metal-to-metal pressure barrier. Although some of the known packoffs with metal-to-metal seals function satisfactorily under certain conditions, there is a growing industry need for such packoffs that can be installed from a remote location without difficulty, that will withstand higher operating pressures than heretofore experienced, and tht will maintain the seal throughout wide fluctuations in pressure.